1. Field of the Invention
The present invention relates to a method for controlling a transmission system of such as a traveling machine, a construction machine, or the like, and more particularly to a method for reducing a shock occuring at the time of a speed change in a transmission system having clutches of a two-stage configuration composed of a main transmission and a sub transmission.
2. Description of the Related Art
In a transmission system having clutches in a first stage (sub transmission) on the input shaft side of a transmission and clutches in a second stage (main transmission) on the output shaft side thereof, a speed is selected through a combination of the sub transmission-side clutches and main transmission-side clutches.
Conventionally, a typical method of effecting a speed change in this type of transmission system is shown in FIG. 7. Specifically, at the time of a speed change, pressure oil is supplied simultaneously (at a timing t.sub.0) to clutches (Low, 2nd) to be set to on among the clutches (High, Low) on the sub transmission side and the clutches (1st, 2nd) on the main transmission side, and the engagement of these on clutches is effected substantially simultaneously (in which case, since the sub transmission-side clutch has a smaller capacity, the sub transmission is normally engaged earlier). That is, in this conventional method, the supply of oil to the two clutches is commenced simultaneously during the speed change.
However, with this conventional method, no problem is presented when the pump discharge is sufficient, but in cases where the pump discharge is smaller than the capacity of the two clutches, the filling time, i.e., the period of torque off is generally prolonged. At the same time, a problem is encountered in that a large speed-change shock occurs due to this torque off and the like, as shown in the part (e) of FIG. 7.
In addition, as another conventional method, one shown in FIG. 8 is known in which the shift on the sub transmission side (High.fwdarw.Low) is effected earlier than the shift on the main transmission side (1st 2nd). With this conventional method, since the pressure of the 1st clutch, i.e., the main transmission-side off clutch, is built at the timing t.sub.1 when the filling of the Low clutch is completed, a speed change from the 2nd speed (H1) 1st speed (L1).fwdarw.3rd speed (L2) is carried out although a speed increase from 2nd speed (H143 3rd speed (L2) should be carried out. Hence, positive and negative speed-change shocks are produced, as shown in the part (e) of the drawing. In this transmission, if the Low clutch, the High clutch, the 1st clutch, and the 2nd clutch are respectively abbreviated as L, H, 1, and 2, then L1, H1, L2, and H2 respectively correspond to the 1st speed, 2nd speed, 3rd speed, and 4th speed.
Furthermore, FIG. 9 shows a case where, in an arrangement in which electronically operated pressure control valves are provided separately for all the clutches H, L, 1, and 2, pressure oil is supplied simultaneously (at a timing t.sub.2) to the main and sub clutches (L, 2) to be set to on, and the sub transmission-side L clutch has completed the filling earlier (at a timing t.sub.3). In this case as well, the same phenomenon as the case shown in FIG. 8 occurs due to a time difference in the completion of filling of the main and sub transmissions.